In recent years, along with the progress in portable or codeless equipments, a demand is mounting for a non-aqueous electrolyte secondary cell which is small in size and light in weight and has a high energy density. As an active material for a non-aqueous electrolyte secondary cell, a composite oxide of lithium and a transition metal, such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Especially, a lithium secondary cell employing a lithium-cobalt composite oxide (LiCoO2) as a positive electrode active material and employing a lithium alloy or a carbon such as graphite or carbon fiber as a negative electrode, provides a high voltage at a level of 4 V and is widely used as a cell having a high energy density.
However, in the case of a non-aqueous electrolyte secondary cell employing LiCoO2 as a positive electrode active material, it was desired to further improve the capacity density per unit volume of the positive electrode layer, and the safety, and further, there was a problem in the uniform coating property for an electrode coating film, a problem of deterioration of the cycle characteristics such that the discharge capacity of the cell gradually decreases as the charge/discharge cycle is repeated, a problem in the weight capacity density, or a problem of large decrease of the discharge capacity at a low temperature.
In order to solve a part of such problems, JP-A-6-243897 proposes that the average particle size of LiCoO2, as a positive electrode active material, is made to be from 3 to 9 μm, the volume occupied by a group of particles having particle sizes of from 3 to 15 μm, is made to be at least 75% of the total volume, and the intensity ratio of diffraction peaks at 2θ=about 19° and 45 as measured by X-ray diffraction using CuKα as a ray source, is made to have a specific value, to obtain an active material excellent in the coating property, self-discharge characteristics and cycle characteristics. The publication further proposes, as a preferred embodiment, LiCoO2 which has particle sizes having substantially no particle size distribution in a range of at most 1 μm or at least 25 μm. With such a positive electrode active material, the coating property and the cycle characteristics are improved, but one satisfying the safety, the volume capacity density and the weight capacity density has not been obtained.
Further, in order to improve the weight capacity density and the charge/discharge cycle characteristics, JP-2000-82466 proposes a positive electrode active material made of lithium composite oxide particles having an average particle size of from 0.1 to 50 μm, and having at least two peaks in the particle size distribution. Further, it is also proposed to obtain such a positive electrode active material having at least two peaks in the particle size distribution, by mixing two types of positive electrode active material having different average particle size. By such a proposal, the weight capacity density of the positive electrode and the charge/discharge cycle characteristics may be improved, but it is cumbersome to prepare two types of positive electrode raw material powders having different particle size distributions, and one satisfying all of requirements for the volume capacity density of the positive electrode, the safety, the coating uniformity, the weight capacity density and the cycle characteristics, has not been obtained.
Further, in order to solve the problem regarding to the cell characteristics, JP-A-3-201368 proposes to replace from 5 to 35% of Co atoms with W, Mn, Ta, Ti or Nb, for improvement of the cycle characteristics.
Further, JP-A-10-312805 proposes to improve the cycle characteristics by using as a positive electrode active material LiCoO2 of a hexagonal system having a crystallite diameter of from 45 to 100 nm in a (110) direction of the crystallite, wherein the length of c axis of lattice constant is at most 14.051 Å.
Further, JP-A-10-72219 proposes that a lithium composite oxide having the formula LixM1−yNyO2 (wherein O<x<1.1, 0≦y≦1), with primary particles being a plate or columnar form, having (“volume base cumulative 95% size”−“volume base cumulative 5% size”)/“volume base cumulative 5% size” of at most 3, and having an average particle size of from 1 to 50 μm, has a high initial discharge capacity per weight and an excellent charge/discharge cycle durability.
However, by the above-mentioned conventional techniques, with respect to a lithium secondary cell using as a positive electrode active material a lithium-transition metal composite oxide, no product has been obtained which fully satisfies all of requirements for the volume capacity density, the safety, the coating uniformity, the cycle characteristics and the low temperature characteristics.
It is an object of the present invention to provide a novel lithium-transition metal composite oxide having a large volume capacity density, high safety, excellent uniform coating property, excellent charge/discharge cycle durability and excellent low temperature characteristics and thus suitable as a positive electrode active material for a lithium secondary cell, a positive electrode for a lithium secondary cell employing it, and a lithium secondary cell.